(1) Field of the Invention
This invention relates to a control system for an automatic transmission, especially to a control system for servo hydraulic pressure adapted to have frictional engagement means engaged or released for changing the gear position of an automatic transmission.
(2) Description of the Related Art
An automatic transmission mounted on a vehicle is generally provided with plural planetary gear mechanisms. Upon shifting the speed, shifting elements (gears, carriers, ring gears) of these planetary gear mechanisms are selectively connected in accordance with engagement or release of frictional engagement means to establish plural speed stages. In such a transmission, friction elements of the respective frictional engagement means are selectively brought into engagement upon shifting from a particular speed stage to a speed stage lower than the particular speed stage, for example, from a 3rd speed to a 2nd speed.
Such a shifting operation as described above has heretofore been conducted using an orifice kick-down valve. Govenor pressure and spring load are applied in a mutually opposing relationship against a spool of the valve. Feeding of servo pressure on an engaging side is effected through a large-diameter orifice at a vehicle speed not higher than a predetermined level but through a small-diameter orifice at a vehicle speed higher than the predetermined level, whereby the friction element on the engaging side is promptly brought into engagement near a point where the number of input revolutions synchronizes with the number of revolutions of an intended speed stage. (see Japanese Patent Publication No. SHO 52-18344).
According to the above-described conventional technique, however, the feeding of servo pressure to the frictional engagement means on the engaging side is effected under the two-step control by the change-over of the orifices irrespective of throttle opening or the extent of drainage of the servo pressure for the frictional engagement means on the releasing side. The point of synchronization therefore varies substantially depending on the vehicle speed and engine characteristics, leading to the problem that the engine may be caused to idle at an excessively high speed or a large shift shock may occur due to excessive tie-up. Further, it is not feasible to correct the point of engagement of the friction element in each frictional engagement means for the elimination of any change in the engaging or releasing speed of the friction element which may occur due to a variation in the stroke of a piston for the friction element, a temperature-dependent change in the fluidity of working oil, a deviation or time-dependent change in the frictional coefficient .mu. of the friction element, or the like. The above conventional technique is accompanied by a further problem, that is, if the frictional engagement means on the engaging side has a large torque capacity while its rotation speed is changing, a sudden speed change and a shift shock may take place due to its tie-up with the frictional engagement means on the releasing side.